Passivation layer

ABSTRACT

An organic light emitting diode device comprises a substrate ( 1 ), a layer ( 3 ) of organic, preferably polymeric, light emitting material, and a transparent cathode ( 4 ) comprising a layer of material with a work function less than 4 eV. The device has a passivation layer ( 5 ) comprising boron oxide.

BACKGROUND OF THE INVENTION

The present invention relates to an organic light emitting diode (OLED)device, a method of manufacturing an OLED device and a passivation layerfor an electronic device.

In particular, the OLED may be a polymer light emitting diode (PLED).PLEDs are usually fabricated on a conductive substrate such as of indiumtin oxide (ITO) forming a transparent anode on to which layers oftransparent conducting polymer, light emitting polymer, and cathodelayers are deposited. A metal can, containing a getter to remove anywater and oxygen, is glued over the device to encapsulate it.

Such a “bottom-emitting” device is expensive and slow to manufacture andis bulky.

Accordingly, “top-emitting” devices are also known, in which thesubstrate is opaque, for example a silicon wafer comprising activecircuitry. In such devices, the light is emitted through the cathode,which must have very good electrical conductivity and transparency.Advantageously the cathode comprises a layer of calcium, e.g. from 5 to30 nm in thickness.

A major problem with such a device is that both the calcium and thelight-emitting polymer are very reactive with oxygen and water. It istherefore known to deposit an encapsulating layer on to the layer ofcalcium to prevent the ingress of oxygen and water. A large number ofpossible materials for the encapsulating layer have been suggested. Forexample, US-A1-20010052752 suggests the use of a dielectric oxideselected from the group consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, MgO,HfO₂, Ta₂O₅, aluminum titanium oxide and tantalum hafnium oxide.Nitrides, such as silicon nitride, have also been proposed.

A serious disadvantage of all of these known materials is that thetechnique by which they are deposited tends to damage the calcium and/orthe light emitting polymer. If the encapsulation material is depositedby electron beam evaporation, secondary electrons oxidize thelight-emitting polymer. If the deposition method is sputtering, bothsecondary electron ionization and heavy ion damage tend to occur. Ifplasma enhanced chemical vapor deposition is used, radiofrequencyelectric fields permeate through the device and, permanently degrade itsperformance. US-A1-20010052752 therefore teaches the use of atomic layerepitaxy as the deposition method, but this is an expensive technique.

It is known to deposit a passivation layer to protect the calcium andlight emitting polymer layers from the subsequent deposition of theencapsulating layer. For example, U.S. Pat. No. 5,739,545 describes zincsulfide as a passivation material. However, the use of zinc sulfide hasbeen found to reduce device lifetime by a factor of 10, possibly becausethe light-emitting polymer becomes contaminated with sulfur.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a practical andeffective passivation layer in a top-emitting OLED.

Accordingly, the present invention provides an organic light emittingdiode device having a passivation layer comprising boron oxide.

We have found that when deposited in a film of suitable thickness, boronoxide (B₂O₃) is effective in protecting the device from subsequentdeposition techniques such as electron beam deposition and sputtering.Importantly, boron oxide can be thermally deposited. Thermal depositiondoes not cause damage to the sensitive light emitting polymer or calciumlayers. Boron oxide also has a very low coefficient of thermal expansion(about 1 ppm/° C. at room temperature) so that the deposited film doesnot crack. This is unusual, since most inorganic salts that can bethermally deposited crack visibly on cooling. Boron oxide appears tohave very few pinholes. Boron oxide films appear to be glassy andamorphous when thermally deposited, unlike most thermally depositedfilms, which are crystalline.

Preferably, the thickness of the passivation layer is from 50 nm to 1μm, and the thickness can be adapted to the energy of the electrons,ions or fields from which protection is required.

Preferably, the device comprises a substrate, a layer of organic,preferably polymeric, light emitting material, and a transparent cathodecomprising a layer of material with a work function less than 4 eV, e.g.calcium. Said passivation layer preferably overlies the layer ofmaterial with a work function less then 4 eV directly.

Preferably, the device comprises an encapsulating layer overlying saidpassivation layer. The encapsulating layer may comprise any suitableencapsulating material, for example a dielectric oxide selected from thegroup consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, MgO, HfO₂, Ta₂O₅, aluminumtitanium oxide and tantalum hafnium oxide.

In a preferred embodiment, the device comprises sealing layers, such asof epoxy resin and glass.

The invention also provides a method of manufacturing an organic lightemitting diode device comprising depositing a passivation layercomprising boron oxide on the device.

Preferably, said passivation layer is deposited by thermal evaporation.

Preferably, the device comprises a substrate, a layer of organic,preferably polymeric, light emitting material, and a transparent cathodecomprising a layer of material with a work function less than 4 eV, e.g.calcium. Said passivation layer is preferably deposited directly on tothe layer of material with a work function less than 4 eV.

In a preferred embodiment, the method comprises a further step ofdepositing an encapsulation layer on to the passivation layer. Theencapsulation layer may comprise any suitable encapsulating material,for example a dielectric oxide selected from the group consisting ofAl₂O₃, SiO₂, TiO₂, ZrO₂, MgO, HfO₂, Ta₂O₅, aluminum titanium oxide andtantalum hafnium oxide. Preferably, the encapsulation layer is depositedby electron beam evaporation, but it may alternatively be deposited bysputtering.

Preferably, the method comprises sealing the device, for example withepoxy resin and glass.

More generally, the invention provides a passivation layer for anelectronic device, the passivation layer comprising boron oxide. As faras we are aware, boron oxide has never been suggested as a passivationmaterial for any application.

BRIEF DESCRIPTION OF THE DRAWINGS

A particular embodiment of the invention will now be described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross section of a device according to theinvention; and

FIG. 2 shows the results of an experiment comparing degradation ofsilicon dioxide and boron oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a top-emitting PLED device comprising a silicon substrate1, a nickel anode 2, a light emitting polymer layer 3 and a transparentcalcium cathode layer 4.

A passivation layer 5 of boron oxide is deposited on the calcium layer 4by thermal evaporation. This process comprises simply heating the boronoxide to evaporate it under a suitable vacuum and is the same processused for depositing the calcium layer 4. Boron oxide evaporates at about1000° C. The thermal evaporation process does not damage the lightemitting polymer layer 3 or the calcium layer 4.

The boron oxide layer 5 is “conformal”, i.e. continuous withoutpinholes. This is demonstrated by FIG. 2, which shows the results of anexperiment comparing silicon dioxide and boron oxide layers. Two testdevices 11, 12, each comprised a glass substrate coated with a thin filmof calcium. The first device 11 was then coated with a layer of silicondioxide whilst the second device 12 was coated with a layer of boronoxide. Both devices were submerged in water. In the first device 11, thecalcium was degraded at pinholes 13. However, in the second device 12,the degradation was uniform, indicating a conformal film of boron oxide.(Boron oxide is slightly soluble in water and cannot thereforeencapsulate on its own.)

Returning to FIG. 1, an encapsulation layer 6 is deposited by electronbeam evaporation on the passivation layer 5. The encapsulation layer isof a suitable encapsulating material such as Al₂O₃, SiO₂, Ta₂O₅ orSi₃N₄.

The device is sealed by a layer of epoxy resin 7 deposited on theencapsulation layer 6, also covering the edges of device layers 2 to 6,and contacting the substrate 1. The device is completed by adding aglass plate 8.

All forms of the verb “to comprise” used in this specification have themeaning “to consist of or include”.

1. An organic light emitting diode device having a passivation layercomprising boron oxide.
 2. A device according to claim 1, comprising asubstrate, a layer of organic light emitting material, and a transparentcathode comprising a layer of material with a work function less than 4eV.
 3. A device according to claim 2, wherein said material with a workfunction of less than 4 eV comprises calcium. 4-8. (canceled)
 9. Amethod of manufacturing an organic light emitting diode device,comprising depositing a passivation layer comprising boron oxide on thedevice.
 10. A method according to claim 9, wherein said passivationlayer is deposited by thermal evaporation. 11-18. (canceled)
 19. Apassivation layer for an electronic device, the passivation layercomprising boron oxide.
 20. A device according to claim 2, wherein saidlight emitting material is a polymeric light emitting material.
 21. Adevice according to claim 2, wherein said passivation layer directlyoverlies said layer of material with a work function less than 4 eV. 22.A device according to claim 1, further comprising an encapsulating layeroverlying said passivation layer.
 23. A device according to claim 22,wherein said encapsulating layer comprises a dielectric oxide selectedfrom a group consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, MgO, HfO₂, Ta₂O₅,aluminum titanium oxide, and tantalum hafnium oxide.
 24. A deviceaccording to claim 1, further comprising sealing layers of adhesive andglass.
 25. A device according to claim 24, wherein said adhesivecomprises epoxy resin.
 26. A method according to claim 9, wherein thedevice comprises a substrate, a layer of organic light emittingmaterial, and a transparent cathode comprising a layer of material witha work function less than 4 eV.
 27. A method according to claim 26,wherein said passivation layer is deposited directly onto said layer ofmaterial with a work function less than 4 eV.
 28. A method according toclaim 9, further comprising a step of depositing an encapsulating layeronto said passivation layer.
 29. A method according to claim 28, whereinsaid encapsulating layer comprises a dielectric oxide selected from agroup consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, MgO, HfO₂, Ta₂O₅, aluminumtitanium oxide, and tantalum hafnium oxide.
 30. A method according toclaim 28, wherein said encapsulating layer is deposited by electron beamevaporation.
 31. A method according to claim 28, wherein saidencapsulating layer is deposited by sputtering.
 32. A method accordingto claim 9, further comprising the step of sealing the device with anadhesive and glass.
 33. A method according to claim 9, comprising thestep of adapting the thickness of said passivation layer to energy ofelectrons, ions, or fields from which protection is required.